In three phase power generation and transmission applications, much effort is expended in order to compensate for loads having less than ideal power factors. In an ideal electrical power distribution system, the transmitted alternating voltage and current should always be in phase. That is, any change in voltage should be accompanied by an instantaneous and proportional change in the current, with both waveforms having their zero-crossings at the same point in time. Such an ideal situation is shown in FIGS. 1A and 1B. FIG. 1B is a rotating vector diagram illustrating the phase relationship between the voltage and current waveforms of FIG. 1A. Since both waveforms are in phase, the voltage and current vectors coincide and there is no angle between them. This is the ideal condition in a power distribution system and occurs when the load that is drawing current from the power distribution system is purely resistive in nature (i.e. no reactive component to the load impedance). Power is transmitted most efficiently when there is no phase angle between the voltage and current waveforms.
In the past, electrical power providers have found that the loads presented to their power distribution systems have been, in the aggregate, inductive in nature rather than purely resistive. Such inductive loading is caused mainly by the transformers used at the power inputs of most electrical equipment. The inductive nature of the load causes the current to lag the voltage, as shown in FIGS. 2A and 2B. In other words, the instantaneous change in the current waveform occurs after some delay in the instantaneous change in the voltage waveform. It can be seen in FIG. 2A that the current waveform 10 is lagging the voltage waveform 12 because the zero crossing of the current waveform 10 occurs at a later point in time than the zero crossing of the voltage waveform 12. As seen in FIG. 2B, this lag is represented by an angle .theta. between the voltage and current vectors. The measure of the degree of current lag is called the power factor, and is expressed as the cosine of the angle n .theta. between the voltage and current waveforms: EQU Power Factor=cosine (.theta.)
The power factor is important in power transmission and distribution because the amount of power transferred to the load depends upon the power factor as follows: EQU Power=V * I cos(.theta.)
The power is maximized by maximizing cos(.theta.). Cos(.theta.) has a maximum value of one when .theta. equals zero. This is why power factor correction is important in power distribution systems. In the case of the inductive load of FIGS. 2A and 2B, the power transfer will be inefficient unless the value of .theta. is minimized (and preferably reduced to zero).
When the load presented to the power distribution system is inductive, a purely resistive load may be simulated by placing a capacitance in parallel with the load, as shown in FIGS. 3A and 3B. The capacitance value C is chosen such that the reactances of the added capacitance and the load's inductance cancel each other, leaving only the resistive component of the impedance. This is illustrated in the rotating vector diagram of FIG. 3B. If I.sub.RESULT is the desired current phase angle (i.e. the same phase angle as the voltage waveform), and I.sub.LOAD is the lagging phase angle caused by the inductive load, the capacitance C is chosen such that when placed in parallel with the inductive load, the leading current I.sub.C is produced. The lagging current I.sub.LOAD will then cancel with the leading current I.sub.C, producing I.sub.RESULT. In such a situation, the voltage and current will be in phase, the power factor will be cos(0)=1, and maximum power transmission efficiency is achieved.
In recent times, power distribution providers have encountered a shift from inductive loading to capacitive loading, mainly due to increased presence of computers and similar devices coupled to the power grid. FIGS. 4A and 4B illustrate the current lead produced by a capacitive load, in which the current waveform 14 leads the voltage waveform 16 by .theta. degrees. The power factor may be corrected in such situations, as shown in FIGS. 5A and 5B, by placing an inductance L in parallel with the capacitance of the load. The inductance L is chosen such that the reactances of the added inductance and the load's capacitance cancel each other, leaving only the resistive component of the impedance. This is illustrated in the rotating vector diagram of FIG. 5B. If I.sub.RESULT is the desired current phase angle (i.e. the same phase angle as the voltage waveform), and I.sub.LOAD is the leading phase angle caused by the capacitive load, the inductance L is chosen such that when placed in parallel with the capacitive load, the lagging current I.sub.L, is produced. The leading current I.sub.LOAD will then cancel with the lagging current I.sub.L , producing I.sub.RESULT. In such a situation, the voltage and current will be in phase, the power factor will be cos(0)=1, and maximum power transmission efficiency is achieved.
Since the aggregate inductance or capacitance of the load is constantly changing as devices coupled to the power grid are turned on or off, the value and type of compensating reactance must be changed frequently. FIG. 6A illustrates a normal contaminated distribution current that results from a combination of inductive loads and capacitor filter rectifier loads. The rectified current is contrasted with an ideal sine waveform. In order to correct the power factor of this type of current waveform with minimum difficulty, most prior art systems use power semiconductor switches to couple or uncouple capacitive or inductive loads into the power distribution system as needed. Such a system is illustrated in FIG. 6B, where an inductance L may be placed in parallel with the load by closing switch SL, and/or capacitance C may be placed in parallel with the load by closing switch SC. When the load is sensed by the system to be capacitive (leading current), the switch SL is closed and the switch SC is opened, thereby placing the inductance L in parallel with the load. On the other hand, when the load is sensed by the system to be inductive (lagging current), the switch SC is closed and the switch SL is opened, thereby placing the capacitance C in parallel with the load.
Such systems have many disadvantages. First, they are typically large and expensive, due to the large amounts of capacitance or inductance that must be made available for switching into parallel with the load. Also, these devices correct the power factor by introducing counter currents that cancel the leading or lagging current drawn by the load. In other words, the power factor corrector attempts to solve the problem by an indirect method (creating additional canceling currents). Therefore the power factor correcting device becomes an additional load, consuming power and increasing the required current generation capacity of the power distribution system.
It will be appreciated by those skilled in the art that the prior art power factor correction methods are cumbersome in that they require large banks of capacitors and/or inductors that may be switched into and out of the load circuit. Additionally, the prior art devices are inefficient because they introduce additional loading into the power distribution grid, requiring more current to be generated. Accordingly, a three phase power factor correction device which overcomes any or all of these problems is highly desirable. The present invention is directed toward meeting these needs.